This invention relates to the removal of SO.sub.x and NO.sub.x compounds from waste gas streams. More particularly, the invention relates to the removal of SO.sub.2, SO.sub.3, NO and NO.sub.2 from industrial stack gases.
The burning of coal or oil as fuel in a boiler or furnace produces a flue gas (or stack gas) usually containing SO.sub.2, SO.sub.3, and NO.sub.x. The concentration of these components in a particular stack gas depends upon such factors as the concentration of sulfur and nitrogen components in the fuel, the metals concentration in the fuel, the air rate fed to the boiler or furnace, and the temperature of combustion. A stack gas, however, will usually contain between about 10 and 2000 ppmv NO.sub.x and between about 100 ppmv and 5 mole percent SO.sub.x compounds, with the large majority of the latter, usually at least about 95% thereof, being in the form of SO.sub.2.
Before a stack gas containing SO.sub.x compounds may be discharged to the atmosphere, many environmental regulatory agencies require that the stack gas be desulfurized, that is, that the concentration of sulfur compounds therein be reduced to specified levels. Similar regulations require the removal of NO.sub.x components in stack gas, largely for the reason that such components contribute to photochemical smog.
The most conventional method presently utilized to remove SO.sub.x compounds from a stack gas involves contacting the stack gas with a liquid absorbent containing dissolved lime or caustic. But although such a process is favorable from a cost standpoint, it is largely ineffective for removing NO, which might also be present in the stack gas. In addition, since the spent absorbent obtained from a lime or caustic treatment is regenerable only by extremely costly techniques, the spent absorbent is usually not regenerated; instead, it is allowed to accumulate for waste disposal, which requires in many instances the daily handling of tons of spent absorbent. Thus, waste disposal of spent absorbent is a nuisance at the least and oftentimes a very difficult task.
Accordingly, it is an object of this invention to provide a process for removing SO.sub.x, and most particularly SO.sub.2, from a gas stream while minimizing the amount of waste products produced. It is a further object to provide a process for (1) effectively removing either or both SO.sub.x and NO.sub.x components from a gas stream by absorption in an aqueous liquid and (2) regenerating the aqueous liquid when laden with dissolved SO.sub.x and NO.sub.x compounds such that said SO.sub.x compounds are converted to hydrogen sulfide and/or sulfur and the NO.sub.x compounds to nitrogen. It is a most especial object of the invention to provide a process for absorbing SO.sub.2 into an absorbent and subsequently regenerating the spent absorbent by converting the absorbed SO.sub.2 to sulfur and/or hydrogen sulfide, with the process having a high efficiency for the overall conversion of the absorbed SO.sub.2 to sulfur and/or hydrogen sulfide. Other objects and advantages of the invention will be apparent in view of the following description of the invention.